1. Field of the Invention
The invention relates to protection from high current flows. More specifically, the invention relates to ground fault protection in cabled signal lines.
2. Related Art
Alternating Current (AC) powered systems are typically electrically wired such that the hot wire and a neutral wire go to a device physically contained within a metal housing. A third wire, the green wire couples to the metal housing, which in turn is coupled to a local ground. Under normal operating conditions, the green wire does not carry current to or from the device. The neutral wire is also connected to local ground but must carry the full current used by the powered device. The neutral wire will exhibit significant voltages due to a current-resistance (IR) drop across the neutral wire. Without the green wire, if the hot wire shorts to the metal housing an electrocution hazard would exist. With the green wire, in the event of a hot wire short to the metal housing large amounts of current will flow through the green wire to ground opening a fuse or circuit breaker in the hot wire circuit.
A consequence of the green wire protection is that the powered device will float its metal housing to the local ground voltage level. However, local grounds can vary by many volts in potential due to nearby equipment returning large currents to earth through their neutral wires, or due to lightning, or due to being in different AC power environments (e.g., between houses the AC power can come from different pole transformers). Electronic equipment makers assume that all powered devices that connect with their equipment have exactly the same green wire environment. This assumption though reasonable is frequently incorrect. Medical, telecommunications, and industrial equipment makers must also deal with the regulations requiring them to limit the chassis-to-chassis current that can flow.
Certain long distance signaling techniques exacerbate the problems and risks created when the above assumption is incorrect. One such technique is serial data transfer using the IEEE Standard for a High Performance Serial Bus, IEEE Std. 1394-1995, published Aug. 30, 1996 (1394-1995). Systems of the genre implementing 1394-1995 or any subsequent update of that standard will be generically referred to herein as 1394 systems. Similarly, 1394 protocol will be used to generically refer to the protocol of 1394-1995 and its subsequent revisions. The high speed serial communications provided for by 1394 protocol is an important input/output (I/O) capability for personal computers (PC's) and consumer electronics peripherals.
A 1394 protocol is a bit serial protocol which uses metal cables to communicate at hundreds of megabits per second. The protocol is defined to permit four and a half meter hops with up to sixteen hops through repeaters permitted. The effect is that distances of 50 meters or more between devices can be expected. Prior experiences with local area networks (LANs) identified that long distance metal connections increase the probability of different ground potentials which can lead to high current flows resulting in melted cables and possible fire. Thus, a liability issue arises by virtue of the use of long distance metal cabling in 1394 systems.
In the 1394 systems the source of the problem is three-fold. First, a possible problem is the power lines. The power lines carry power from some external power supply, and that power supply is usually tied to the green wire local ground at its location. Thus, if there is a vastly different local ground potential between the power supply and a supplied device, the power lines may carry a large current independent of the power being supplied.
The second problem is the signal lines themselves. The 1394 cable typically contains two differential pairs of signal lines which are small gauge and carry high frequency signals to silicon transceivers at either end. The silicon transceivers, of course, are powered and have a ground reference. If either of the transceivers is not supplied by an isolated power supply or if either of the transceivers is intimately or directly connected to the logic ground of the remainder of the circuit, then a sneak path to local ground exists through the signal wires. If the local ground potential between the transceivers is sufficiently different, high current will flow in the signal wires, destroying the transceivers and resulting in a highly conductive path, cable melting and possible fire.
The third problem is that 1394 systems require high speed signaling coming from a noisy environment. Accordingly, to avoid picking up external radiation from transmitters and to meet Federal Communication Commission (FCC) admissions guidelines, metal shielding must be provided around the cable. The metal shields are connected to the chassis at either end of the cable. Inasmuch as the metal shields conduct and the chassis may have different ground potentials, a shield melt-down may result.
Each of these problems has long been identified and 1394-1995 has a proposed solution for each. However, as discussed below, economic and technical impediments to the implementation of these proposed solutions makes wide spread adoption unlikely. Moreover, because of requirements imposed by the protocol on good citizens, implementation by some devices in the network but not all will not insulate the implementer from potential liability. For example, a printer manufacturer may implement all possible costly procedures yet still find its printer in a mass of melted cables making innocence difficult to prove.
With respect to the power supply, 1394-1995 requires that manufacturers isolate the power supply. This implies transformers with separate windings and a separate ground plane such that the ground of the transformer is not tied to the green wire local ground. Unfortunately, these special steps are quite expensive, so as a practical matter, cheap devices will not isolate the power supply and rather they will continue to tie to the local ground. That being the case, it does not matter what steps other devices in the network have taken in terms of isolation; since at least one device in the network did not properly isolate, a melt down risk exists. This is particularly true since the guidelines require that all multi-port devices pass through power. Therefore, the quality device (such as the printer in the example above), having taken all the requisite steps to isolate its power supply and even if not supplying power itself, is required to pass through the power supplied by an unisolated device.
With respect to the signal wires, 1394-1995 requires that the transceiver for the device be isolated. This requires capacitive or transformer blocking to be placed between the transceiver data path and its controlling link. This transceiver isolation is consistent with what is done with ethernet. Unfortunately, this isolation of the transceiver currently carries an incremental cost which may be $5.00 or higher. Whereas, with current processing technology, the transceiver can be integrated onto a single chip with the device at very low cost. Accordingly, economic considerations will ultimately drive device manufacturers to integrate and isolation will be lost.
With respect to shielding, 1394-1995 proposes that the shield not be tied to the bulkhead, chassis, or Faraday cage (generically metal housing) of the device directly but rather be tied to the metal housing through a capacitor with a one megaohm resistor. The resistor bleeds off static electricity and the capacitor is chosen to block 60 Hz power signals but permit the high frequency signals to be passed. Unfortunately, using a single capacitor results in series inductance in the long leads which does not result in a good radio frequency (RF) ground. Moreover, because all the current from the 360.degree. around the shield is routed through a single channel at the capacitor, a slot antenna is created which acts as a radiator in violation of FCC guidelines. Thus, this fix is not commercially viable.
For economics and feasibility reasons, these solutions are not suitable for future generations of 1394 systems. The net result of the foregoing is that subsequent editions of the 1394 Standard specifically, P1394 a Draft Standard for a High Performance Serial Bus (1394a) and P1394 b Draft Standard for a High Performance Serial Bus (1394b) have eliminated all isolation and ground fault protection requirements. But because of the potential liability these problems represent, unless a commercially viable resolution can be found many manufacturers will be hesitant to assume the risks of supporting 1394 protocol. Accordingly, it would be desirable to develop a method and apparatus to limit the risk of catastrophic failure resulting in possible fire, shock hazard, and potentially high liability.